Photovoltaic cells produce useful electrical energy by the absorption of incident radiation and the generation of electrons and holes. The subsequent separation of the electrons and holes in a built-in electric field, produced for example by a P-N junction, results in the generation of a photovoltage and a photocurrent in an external circuit. The output voltage of a photovoltaic cell is limited by the incident radiation intensity and the materials comprising the cell to some maximum value, typically between 50 and 60 percent of the smallest bandgap energy of the semiconductor materials comprising the cell. With a larger cell area the output current increases but not the output voltage. To increase the output voltage, a plurality of these cells have been electrically connected in series, forming a photovoltaic array.
Photovoltaic cells composed of materials such as amorphous silicon, as described by Carlson in U.S. Pat. No. 4,064,521, incorporated herein by reference, are used in series-connected arrays since they can be fabricated as thin layers. The deposited layers are sub-divided into a plurality of cells as disclosed, for example, by Hanak in U.S. Pat. No. 4,316,049, incorporated herein by reference, using well-known techniques such as photolithography and chemical etching, mechanical scribing or by laser scribing as disclosed by Hanak in U.S. Pat. No. 4,292,092, incorporated herein by reference. Laser scribing of the deposited layers is presently preferred because of its cleanliness and manufacturing flexibility.
In the prior art the fabrication of a thin layer photovoltaic array included the steps of depositing isolated conducting segments on an insulating substrate, depositing a semiconductor body having a rectifying junction over the segmented conducting layer and the channels therebetween, and forming a continuous, linear opening through the body close to and parallel to an edge of each segment. An electrical contact layer is then deposited over the semiconductor body and on the conducting segments in the openings in the semiconductor body. The contact layer is then sub-divided by means of grooves adjacent to the linear opening in the semiconductor body.
These series connections, while simple and useful in mass production, are often not sufficiently conducting. The underlying segment is also sometimes severely damaged by the scribing process which markedly increases the series resistance. If the segment is completely severed the series contact is lost and the array is useless.
Alternatively, if the openings in the semiconductor body and the contact layer are formed simultaneously, only one side of the scribe line may contribute to the electrical connection between the layer and a segment. The other side of the scribe line is prevented from acting as an electrical contact because the metal electrode is severed.
It is desirable to have an interconnection scheme which maintains the advantages of the prior art methods but which improves the contact between adjacent cells.